Move all scripts and documentation to Python3 (#6378)

* Move all scripts and documentation to Python3

Python 2 EOL is Jan 1, 2020.  Migrate scripts to run under Python 3.

Under Windows, we're already running Python 3.7, by dumb luck.  The
oddness is that the Windows standalone executable for Python 3 is called
"python" whereas under UNIX-like OSes it's called "python3" with
"python" always referring to the Python 2 executable.  The ZIP needs to
be updated to include a Python3.exe (copy of Python.exe) so that we can
use the same command lines under Linux and Windows, and to preserve my
sanity.

Fixes #6376

* Add new Windows ZIP with python3.exe file

* Sort options in boards.txt generation for repeatability

The order of the board opts dict changes depending on the Python version
and machine, so sort the options before printing them to get a stable
ordering.

* Re-add Python2 compatibility tweaks

Most scripts can run as Python 2 or Python 3 with minimal changes, so
re-add (and fix, as necessary) compatibility tweaks to the scripts.

package/drop_versions.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # This script drops one or multiple versions of a release
 #
 from __future__ import print_function